The present invention relates to a monitoring system of an optical power in an optical repeater utilized in an optical communication system, in particular, relates to such a system in which the tele-monitoring of an optical power in an optical repeater is possible from a terminal station on land.
In an optical communication system which utilizes an optical fiber as a transmission line, when there is something wrong with a transmission line and/or a repeater, the fault point must be quickly located and must be repaired. Therefore, the operation of the transmission line and/or repeaters must be tele-monitored in terminal stations on land.
When an optical transmission system is installed on land, an interstitial wire which is made of conductive metal wire is attached along the optical transmission line for the transmission of the monitoring signal of the repeaters. The test and/or the control of repeaters is performed through that interstitial wire to locate the fault point.
When an optical transmission system is installed in the seabed, that interstitial wire is not desirable in view of the complicated structure and/or the reliability of a submarine transmission cable. FIG. 1 shows the prior monitoring system for repeaters in a submarine cable. In the figure, the reference numeral 1 is the upward transmission line, 2 is the downward transmission line, 3 is a repeater in the upward direction, 3a is a repeater in the downward direction, 4 is an optical fiber transmission line, and 5 is a return path selectively provided between an upward repeater 3 and a downward repeater 3a. Also, T.sub.1 and T.sub.2 are terminal stations installed on land. A set of repeaters 3 and 3a and a return path 5 are mounted in a single housing of a repeater. The terminal station on land controlls that return path of the selected repeater to close when that repeater is to be tested, and provides the circular transmission path through the downward transmission line 2, the repeater 3a, the return path 5, the repeater 3, and the upward transmission line 1. Then, the terminal station transmits the test signal in that circular transmission path. Therefore, the test signal travels in that circular transmission path and returns to the terminal station. Then, that terminal station compares the received signal with the transmitted test signal, and determine the code error rate in that circular transmission path. In this case, if the return path 5 is provided in each repeater alternately, the difference of the code error rates can be obtained, and then, from that difference the repeater in the fault can be located.
By the way, the most important element in an optical repeater so far as the failure is concerned, is an optical power source which converts the electrical energy to the optical energy. That converted optical energy is transmitted into an optical transmission line. That optical power source is usually implemented by a laser. Accordingly, a monitoring system which monitors the bias current applied to the laser in a repeater has been proposed for an optical transmission system on land. FIG. 2 shows the block diagram of that monitoring system, and in the figure, the reference numeral 4 is an optical fiber, 6 is an optical power source or a laser, 7 is an APC control, 8 is a modulator, 9 is an interstitial wire for transmitting the monitored result, 10 is a bias current in the laser 6, and 11 is a power amplifier. In FIG. 2, when the laser 6 is degraded, the bias current is increased. Therefore, by monitoring the bias current applied to the laser, the operation of the laser and/or the failure of the same is detected.
However, that system shown in FIG. 2 has the disadvantages shown below.
(1) The monitored data is indirect, since the power of a laser is not measured, but only the bias current of the same is measured.
(2) The monitored result is transmitted through an interstitial wire.
(3) The modulator 8 which is actually a voltage-frequency converter, is complicated, and has low reliability.